FIG. 1089. Diagrams illustrating the development of the foetal membranes of a Bird. A, 

 early stagein the formation of the amnion, sagittal section ; B, slightly later stage, trans- 

 verse section ; C, stage with completed amnion and commencing allantois ; D, stage in 

 which the allantois has begun to envelop the embryo and yolk-sac. The ectoderm is 

 represented by a blue, the endoderm by a red line ; the mesoderm is grey. all. allantois ; 

 all', the same growing round the embryo and yolk-sac ; am. amnion ; am./., aw./', amniotic 

 fold ; an. anus ; br. brain ; cod. coelome ; cod', extra-embryonic coelome ; ht. heart ; ms.ent. 

 mesenteron ; mth. mouth ; nch. notochord ; sp. cd. spinal cord ; sr.m. serous membrane ; 

 umb.d. umbilical duct ; vt.m. vitelline membrane yk. yolk-sac. 



